The present invention relates to an optical pickup system for recording or reproducing data on or from an optical disc.
The optical disc such as the CD and the DVD (Digital Versatile Disc) has the recording and reproducing plane between a transparent substrate and a hard protective layer. Data are recorded or read by applying a laser beam to irradiate the recording and reproducing plane passing through the transparent substrate.
Meanwhile, the thickness of the transparent substrate is not always regular in the entire surface thereof because of manufacturing errors. Generally the error is several ten xcexcm. The variation of the thickness of the transparent substrate causes the spherical aberration at a spot of the laser beam, which reduces the accuracy in recording and reading of information.
Heretofore, the numerical aperture of the objective for focusing the laser beam on the recording and reproducing surface of the disc is adjusted so as to reduce the influence of the spherical aberration.
In recent years, there is required the increase of the recording density with the increase of the information quantity. It is accordingly thought to further increase the numerical aperture of the objective to reduce the diameter of the spot.
However, if the numerical aperture is increased, the allowability to the spherical aberration decreases. As a result, it is difficult to realize the high density recording and the reproducing of the information recorded at a high density.
An object of the present invention is to provide an optical pickup system which may detect the thickness difference of a disc without reducing the allowability to the spherical aberration.
According to the present invention, there is provided a pickup system for an optical disc having a transparent substrate which has a front surface and a recording and reproducing plane opposite to the front surface, the system comprising, at least one light source for emitting a light beam, light condensing means for applying the light beam from the light source to the recording and reproducing plane and to the front surface of the transparent substrate, at least one photodetector for detecting return light beams from the recording and reproducing plane and the front surface, first signal generating means for generating a first signal based on an output signal of the photodetector dependent on the return light beam from the recording and reproducing plane, second signal generating means for generating a second signal based on an output signal of the photodetector dependent on the return light beam from the front surface, and signal processing means for producing a signal relating thickness information on a thickness of the transparent substrate by comparing the first and second signals with each other.
The light condensing means further has a first area for applying the light beam from the light source to the recording and reproducing plane and a second area for applying the light beam from the light source to the front surface.
The light condensing means has a first area for transmitting the return light beam from the recording and reproducing plane and a second area for transmitting the return light beam from the front surface.
The second area is provided within the first area and transmits the return light beam from the front surface.
These and other objects and features of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.